The announcement by al-Qaida's ruling council, made through an affiliated Islamist website, marks a major attempt to re-assert its authority six weeks after the US special forces raid that killed Bin Laden in northern Pakistan.

But questions remain about whether Zawahiri, considered more argumentative and less charismatic than the Saudi, can unite al-Qaida's various factions across south Asia and the Middle East while evading his American pursuers.

The 59-year-old has also officially replaced Bin Laden as the world's most wanted fugitive, with a $25m reward for his capture. The next largest US bounty, $10m, is for the Afghan Taliban leader, Mullah Omar.

Today's appointment marks the end of a six-week consultation headed by Saif al-Adel, head of al-Qaida's ruling council, who served as interim leader. Yemen is now considered the most powerful faction, although al-Qaida remains headquartered along the mountainous Afghanistan-Pakistan border.

Bin Laden's deputy had already taken on the mantle of leader, and in a recent video message addressed the concerns about his leadership qualities, adopting a distinctly inclusive tone.

Dressed in a white tunic and sitting beside a gun, Zawahiri called on al-Qaida affiliates to "continue along the path of jihad" while warning America it faced "a rebelling nation which has awaken from its sleep in a jihadist renaissance".

"It was a complete change of language," said Palestinian journalist and militancy expert Baker Atyani. "He's no longer giving instructions or going into detail but talking in general terms, like a leader, as Bin Laden used to."

From a family of wealthy doctors and religious scholars in Cairo, Zawahiri met Bin Laden in Saudi Arabia in 1986 and helped him found al-Qaida in the Pakistani city of Peshawar two years later.

The US later indicted him for his role in the August 1998 bombings of US embassies in Tanzania and Kenya, which killed 224 people and wounded several thousands.

After 2001 he became the most visible face of al-Qaida, appearing in video messages much more frequently than Bin Laden, spouting fiery rhetoric.

"He likes to talk, to argue, to discuss. Bin Laden was very quiet, he would count his words," said Atyani, who met both men near Kandahar in June 2001.

Long been thought to be hiding in Pakistan's tribal belt, he escaped death in a 2006 drone strike on a house in Bajaur that killed 18 people. Other reliable reports over the past decade have placed him in north and south Waziristan.

But since the discovery of Bin Laden in Abbottabad, a garrison town 35 miles north of Islamabad, experts believe Zawahiri may also be sheltering outside the tribal belt. "The border is too dangerous for anyone right now," said Ahmed Rashid, author of several books on militancy.

But his greater problems may be from within, Rashid added. He said: "Osama was much better at placating the various wings of al-Qaida – the Yemenis, the Somalis, the North Africans. I'm not sure Zawahiri can do that."

Efforts to capture him may be hampered by growing distrust between Pakistan and the US. Yesterday it emerged that Pakistani intelligence had captured five people accused of helping the CIA run a secret operation to catch Bin Laden.

Humiliated and angered by the US raid that killed Bin Laden, Pakistan's generals want to stop the CIA drone campaign in the tribal belt. Pakistan has reportedly stopped supplies entering the remote base where the drones are stationed, and the US has moved some across the border to Afghanistan.

Four of the six intelligence "fusion centres", where the CIA and Pakistan's Inter-Services Intelligence share information, have been closed, according to the Washington Post. Any further deterioration in relations between the spy agencies could seriously damage efforts against al-Qaida.

In his last statement, Zawahiri warned that al-Qaida was preparing to mount an attack similar to "black Tuesday" – a term for the 9/11 attacks. Whether he can pull off such a terrorist spectacular will determine both the strength of al-Qaida and Zawahiri's relevance following the loss of Bin Laden.

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